Separation of wheat flour into vital gluten and starch is an important industrial process. The isolated gluten proteins have diverse applications, e.g. as texturising or adherence agents, as wheat flour improvers in the bread-making industry, while the starch produced is processed into sugar syrups or used in industrial applications in general and the food industry in particular.
Industrially such gluten-starch separation processes always involve at least one wet separation step, such as the conventional “Martin process”. In the Martin process, wheat flour and water are mixed into a dough and the starch is extracted from the dough by washing with water.
Another example of a process involving a wet separation step is disclosed in EP A 090.533, wherein in a first step wheat flour is mixed with water to obtain a dough. The gluten aggregates formed during mixing are then washed while gently kneading. Starch is separated from the slurry by this washing step or by centrifugation with decanters or hydrocyclones, and the non-soluble gluten are recovered from the slurry by sieving followed by drying over a ring drying using air of about 100°-150° C. In case of flash drying higher temperatures up to even 300° C. are often applied. The drying step is very critical, since gluten can easily be damaged by severe heating and lose its functionality. Typically, gluten resulting from these processes have a protein content of 75-80%, based on dry matter.
A disadvantage of these separation processes is that copious amounts of water are used that need to be purified afterwards. Secondly, the gluten will always suffer from a certain amount of heat damage. Thirdly, inherent to the current process is the loss of soluble proteins, which are washed away, and losses through gluten-starch and gluten-NSP complexes probably formed during kneading. These gluten-NSP complexes cannot be purified to vital gluten with current technologies. Fourth, overkneading is a known problem with prior art processes and often results in lower yields. Additionally, in a continuous process, it is difficult to prevent overkneading and often requires the selection of wheat of high quality which leads to higher raw material costs. In other words, since the prior art process heavily relies on optimal mixing of the dough, the mixing step has to be adjusted to compensate for differences in flour quality. Also, the prior art process seems less suitable (lower yield) for flour of poor breadmaking quality.
A further drawback is the requirement of a kneading step, which is recognized in the art to influence the amount of an important fraction of glutenin aggregates, referred to as glutenin macro polymer (GMP). Whereas kneading of dough is essential to establish an initial wet separation, prolonged kneading times disadvantageously results in a significant part of the gluten being lost in an unrecoverable gluten-polysaccharide complex. Besides, it should be noted that the effect of kneading time on product quality is dependent on the flour quality used.
EP A 010.447 teaches a method where agglomeration of gluten is achieved by dilution of a fully developed dough obtained by mixing 0.6-1 part water per part by weight of flour with a conventional mixer, further diluting this dough with 0.5-3 parts of water per part by weight of dough and either simultaneously or subsequently applying shear to the mixture, whereby the gluten agglomerates. Recalculating on shows that the inventors doughs with a moisture content varying from 0.58-0.88 based on dry weight of the flour. Shearing is applied by e.g. heavy agitating and/or pumping the dough water mix through a narrow orifice. According to T. R. G. Jongen et al. Cereal Chem. 80, 383-389 (2003), this mixing process is somewhat confusingly called “shearing”, but actually consists of a combination of three components, i.e. shear flow, rotational flow and elongational flow. In the article of Jongen et al., a scalar parameter D is used to distinguish pure rotational flow (D=−1), pure shear flow (D=0) and pure elongational flow (D=+1) which will be discussed in more detail below. Consequently, the process according to EP A 010.447—together with the first kneading step to obtain the dough—is indifferent from conventional kneading processes. Moreover, the additional dilution step and the relatively high water temperature of 40°-50° C. are believed to reduce the product quality even further.
Furthermore, EP A 282.038 discloses a process for the preparation of a water-insoluble, modified gluten product that is obtained by (a) kneading a mixture of wheat flour, L-ascorbic acid, cystine and water, (b) mixing the dough as obtained in step (a) under a high shear force, preferably in an extruder, a meat-chopper or a machine which can mix dough under a high shearing force (cf. EP A 282.038, page 5, line 28), wherein the gluten are mechanically broken down into smaller molecules and the content of proteins soluble in 0.05 N aqueous acetic acid is increased to an amount of 75 to 85 wt. %, based on the total protein content of the dough, and (c) separating the water-insoluble, modified gluten from the dough. However, mixing processes involving high shear consists of the combination of the three components shear flow, rotational flow and elongational flow as described above. The process according to the present invention, however, only involves shear flow which is defined in this document as “simple shear” or “simple shear flow”. In addition, the process according to the invention provides gluten products having properties different from the gluten products known from the prior art.